1. Field of the Invention
This invention relates to deformable mirrors in general, and more particularly to deformable mirrors having replaceable electrodistortive actuators for controlling the shape of the mirror's reflecting surface.
2. Summary of the Prior Art
It is well known in the prior art to use deformable mirrors to correct optical signals for aberrations introduced into a wavefront due to the transmission of an optical signal through a distorting medium, e.g. the Earth's atmosphere. Such deformable mirrors may be used, for example, to correct aberrations introduced into laser signals or optical images imaged by a telescope as they pass through the Earth's atmosphere, or to introduce known aberrations into a laser signal prior to its transmission through the Earth's atmosphere. Examples of deformable mirrors which may be used to perform the foregoing function appear in U.S. Pat. Nos. 3,904,274 and 4,657,358 which are owned by the assignee of the present invention and the teachings of which are incorporated herein by reference. Mirror's of the foregoing type employ a plurality of actuators, for example piezoelectric actuators, which are independently operable to selectively deform areas of a deformable reflecting surface to correct known anomalies contained in an impinging optical signal's wavefront. Signals for driving the actuators to correct the aberrated wavefront may be provided by a wavefront sensing and correction system such as that described in U.S. Pat. No. 3,923,400, the teachings of which are also incorporated herein by reference.
As noted in U.S. Pat. No. 4,657,358, actuators for use in controlling a deformable mirror may be manufactured from stacks of electrodistortive material, for example lead magnesium niobate (PMN) or lead zirconate titanate (PZT). The electrodistortive material is interleaved in the stack with alternating layers of electrical conductors to permit the application of an electrical signal to the material.
Prior known deformable mirrors which utilize stacked electrodistortive material to provide control of the mirror's reflecting surface have been constructed with each actuator rigidly fastened to the member containing the mirror's reflecting surface to permit the reflecting surface to be drawn back by the actuators into an unbiased (or rest) position. Such construction has not provided for ease of repair of a deformable mirror in the event of the failure of one or more actuators. In the event of the failure of an actuator due, for example, to the mechanical failure of an actuator or to the failure of electrical contacts between one or more of the layers of electrodistortive material, the components of the mirror had to be carefully disassembled to effect the removal and replacement or repair of the failed actuator. The disassembly procedure is usually time consuming, frequently difficult, and often impossible to effect due to the spacing between adjacent actuators and the need to prevent damaging good actuators during the repair and reassembly procedure. Consequently, there is a need for a deformable mirror which utilizes electrically operated actuators which may be easily replaced without damaging other actuators during the replacement procedure.
In a related patent application, Ser. No. 266,616 filed Nov. 3, 1988, owned by the owner of the present patent application and the teachings of which are incorporated herein by reference, a construction for a deformable mirror is disclosed in which the mirror's electrodistortive actuators may be easily removed from the mirror's base or support structure for purposes of calibrating and/or replacing one or more of the mirror's actuators. Each actuator includes a threaded portion which is received in a threaded aperture in the base. The position of each actuator with respect to the mirror's deformable reflecting surface may be adjusted by screwing the actuator into its threaded aperture until the top of the actuator just contacts the structure supporting the reflecting surface without deflecting the surface. Further adjustment of the position of each actuator with respect to the mirror's reflecting surface may be used to adjust the initial (or rest) shape (i.e., the extent of deformation) of the mirror's reflecting surface.
While a mirror having replaceable actuators as described above has the advantage of permitting the replacement of one or more of the mirror's actuators, to take advantage of such construction the mirror is required to have a flexible membrane attached to the mirror's faceplate to maintain the contact between the faceplate and the actuators which would normally be supplied by mechanically fastening the actuators to the faceplate. The membrane must be carefully designed and manufactured to insure that a precise downward pressure is applied by it against each actuator to prevent the membrane from introducing undesirable pressure variations and, hence, unwanted surface deformations across the mirror's reflecting surface. The present invention provides an advantage over prior known forms of deformable mirrors in that it avoids the necessity of either mechanically fastening the actuators to the mirror faceplate or the using springs or the like to retract the faceplate. Furthermore, it also avoids the need to provide a reverse electrical bias to the electrodistortive actuators to effect retraction of a deformable mirror's reflecting surface.